Since an aluminum alloy exhibits a lightweight nature and a high thermal conductivity, and since a high corrosion-resistant feature can be realized in the aluminum alloy by a suitable processing, it is utilized in a heat exchanger for an automobile, for example, a radiator, a condenser, an evaporator, a heater, an intercooler and so forth. For a tube member for the automobile heat exchanger, a 2-ply clad material or a 3-ply clad material is used: the 2-ply clad material includes a core material composed of an Al/Mn-based alloy such as JIS3003 alloy and so forth, and an Al/Si-based filler material or an Al/Zn-based sacrificial anode material cladded on one surface of the core material; and the 3-ply clad material includes a core material composed of an Al/Mn-based alloy such as JIS3003 alloy and so forth, an Al—Si-based filler material or an Al/Zn-based sacrificial anode material cladded on one surface of the core material, and an Al/Si-based filler material cladded on the other surface of the core material.
Usually, in a heat exchanger, one of these clad materials is combined with and joined to a corrugated fin by carrying out a brazing process at a high temperature on the order of 600 C, to thereby produce a tube member. After the tube members so produced are installed as a heat exchanger in an automobile, when any one of the tube members is broken and pierced, the tube member concerned leaks a cooling water or a cooling medium circulating in an interior of the heat exchanger. Thus, in order that a service life of the heat exchanger can be prolonged, there is a need of an aluminum alloy brazing sheet exhibiting a superior strength after being subjected to a brazing process.
Incidentally, in recent years, there is much demand of lighter weight for an automobile, and thus a heat exchanger for the automobile must become lighter to comply with the demand. To this end, it has been considered and studied that members for producing the heat exchanger are made thin, and thus it is necessary to further improve a strength of the aluminum alloy brazing sheet after being subjected to the brazing process.
Conventionally, in general, for a tube member for producing a heat exchanger, such as a radiator or a heater for automobiles, in which a coolant is circulated, a 3-ply tube member has been used, with the 3-ply tube member including a core material composed of an Al/Mn-based alloy or the like, which is represented by JIS 3003, an inner surface of the core material being cladded with a sacrifice material such as Al/Zn-based alloy or the like, an atmosphere-side surface or outer surface of the core material being cladded with a filler material such as Al/Si-based alloy or the like. Nevertheless, a strength of the clad material, in which the core material composed of JIS 3003 alloy is used, is on the order of 110 MPa after it is subjected to a brazing process, and this strength is insufficient.
In general, it is considered that the core material composed of the Al/Mn-based alloy, which covers a major part of the strength of the brazing sheet, is a dispersion strengthened alloy. For this reason, in the prior art, it has been proposed that the core material is strengthened by making the density of particles of intermetallic compounds in the core material high, and making the particles of intermetallic compounds in the core material small. Nevertheless, since the brazing sheet is heated to a temperature of 600 C, a major part of the fine intermetallic compounds is again subjected to a solid solution. Thus, in order to improve the strength of the core material, it is significant to reduce the coarse intermetallic compounds which are not again subjected to the solid solution during the brazing process, resulting in a large contribution to a solid solution strengthening effect. Since the coarse intermetallic compounds are mainly produced during various processes between a casting process and a hot rolling process, it is necessary to minutely control the conditions of these processes.
For example, in Patent Document 1, it is disclosed that a core material is subjected to a homogenization process at a temperature of at least 570° C. over a time period of at least 8 hrs, that the core material is then subjected to a hot rolling process at a temperature within a range of 450-550° C., and that various conditions of other processes are minutely regulated. As a result, a metal texture so obtained features that a density of precipitates having a size of at least 1 μm is at least 10,000/mm2. Nevertheless, no reference is made to the fact that the intermetallic compounds are again subjected to the solid solution during the brazing process.